from their surfaces and deposit them in the form of definite coherent layers or membranes. Similar membranes may also be formed in the cell-interior. Of these, the best known is the nuclear membrane. Hence, in considering the general organization of the cell, cytoplasm and nucleus are usually described as bounded by definite structurally distinct layers, plasma-membrane and nuclear membrane. Vacuole-membranes, sphere-membranes and plastid-membranes may also exist in certain cells. To all of these structures it has been customary to ascribe a more or less mechanical or simply protective or isolating function. On the other hand, many cells show no optically distinguishable membranes, either at their surfaces or in their interior; certain ameboid cells and the blood-corpuscles of vertebrates are apparently without membranes and are often described as "naked masses of protoplasm." Yet in such cases the nakedness is only apparent, for it can readily be shown that these cells have membranes which are highly definite in character, but whose existence can be demonstrated only by certain forms of physiological experimentation.
The membranes whose physiological rôle forms the subject of this article are not to be identified with those more or less conspicuous layers separated at the surfaces of many animal and plant cells. The cellulose membranes of plant cells and the various cuticular structures of animal cells are dead structures, whose function is typically passive and mechanical. They are to be sharply distinguished from the membranes about to be considered, whose rôle is a characteristically active one, and, as I believe, fundamentally important in the life of all cells. These membranes are present in all living cells without exception, whether a visible external layer is present or not. Thus red blood corpuscles, though typically naked cells, show by their behavior in salt-solutions of varying concentration that they are bounded by a difficultly permeable surface-layer which is different in its physical properties from the internal protoplasm—having in fact the essential properties of a semi-permeable membrane. Plant cells, like those of Spirogyra, also behave in such solutions as if the surface-layer of the protoplasm were semi-permeable; the visible cellulose membrane plays no part whatever in the osmotic process (plasmolysis) observed under such conditions, while the invisible surface-film of the protoplasm is all-important. Hence in the case of plant cells the conceptions of cell-membrane—i. e., the hardened secretion of cellulose—and of plasma-membrane—or semi-permeable surface layer of the living protoplasm—have to be kept sharply distinct. It is the plasma-membrane, the most external layer of the living protoplasm, with which I shall be chiefly concerned in the present article, and I propose to discuss briefly various questions which arise in reference to this structure: what is its physical and chemical nature? what are the conditions of its formation? and how does it influence the characteristic activities of the cell?
